In recent years, thermal transfer systems have been developed to obtain prints from pictures that have been generated from a camera or scanning device. According to one way of obtaining such prints, an electronic picture is first subjected to color separation by color filters. The respective color-separated images are then converted into electrical signals. These signals are then transmitted to a thermal printer. To obtain the print, a cyan, magenta or yellow dye-donor element is placed face-to-face with a dye receiver element in an image assembly. The two are then inserted between a thermal printing head and a platen roller. A line-type thermal printing head is used to apply heat from the back of the dye-donor sheet. The thermal printing head has many heating elements and is heated up sequentially in response to one of the cyan, magenta or yellow signals. The process is then repeated for the other colors. A color hard copy is thus obtained which corresponds to the original picture viewed on a screen.
Dye receiver elements used in thermal dye transfer generally include a support (transparent or reflective) bearing on one side thereof a dye image-receiving layer, and optionally additional layers, such as a compliant or cushioning layer between the support and the dye receiving layer.
Various approaches have been suggested for providing a thermal dye receiving layer. Solvent-coating of the dye receptive polymers is a commonly used approach. Such methods involve expensive, polluting, and hazardous manufacturing processes. To reduce risks of fire, explosions, and other accidents, special precautions and expensive manufacturing apparatus are needed for handling the organic solvent solutions used in that type of manufacture. Another approach involves hot-melt extrusion of the dye receiving polymers onto a support. Such methods restrict the type of materials that can be incorporated into the layer due to the high temperatures required for the extrusion process. Still another approach utilizes aqueous coating of water-soluble or water-dispersible polymers to provide the dye receiving layer.
Although such aqueous coating methods reduce or eliminate the use of hazardous solvents, and high temperature coating processes, such aqueous-coated layers cause problems in typical customer printing environments where high speed printing requires a smooth separation of donor ribbon element and receiver element with no sticking between the two surfaces. Printing in high humidity environments can be particularly troublesome for sticking with typical aqueous-coated receivers. Moreover, such receiver elements are often deficient in providing adequate dye density. Furthermore, imaged prints bearing the aqueous coated layer are not robust in situations where the print is contacted with water and separation of the layer can occur.
Thus, a common problem with the use of some thermal dye donor elements and corresponding thermal dye receiver elements is that at high dye transfer temperatures, the polymers in the elements can soften and cause adherence between the elements, resulting in sticking and tearing of the elements during separation. Areas within the donor element (other than the transferred dyes) can adhere to the receiver element, rendering the receiving element useless.
This problem has been addressed in many ways including the incorporation of release agents such as silicone waxes and oils as lubricating materials in either or both elements. For example, U.S. Pat. No. 5,356,859 (Lum et al. describes the use of dimethyl siloxane in thermal dye image receiver elements and U.S. Pat. No. 4,962,080 (Watanabe) describes the use of alcohol-modified silicone oils in a similar manner.
U.S. Pat. No. 7,189,676 (Bourdelais et al.) describes an image receiver sheet comprising a crosslinked co-polymer of polyester and a lubricating polymer comprising a polyurethane wherein the crosslinked copolymer is formed from a water dispersion. Such copolymers are difficult to synthesize and are rarely commercially available. U.S. Pat. No. 5,529,972 (Ramello et al.) describes an image receiver sheet with a dye receiving layer comprising a dried polymeric latex wherein the latex may be selected from a group including polyurethane latexes. The technology as described in this patent does not provide adequate maximum densities. In addition, a separate layer of siloxane material is coated above the receiver layer to provide protective and release properties. This requires an additional manufacturing operation. U.S. Pat. No. 4,962,080 (Watanabe) describes an image receiver sheet with an aqueous dye receiving layer, wherein the receiver layer also comprises silicone oil. This patent shows that very low densities are obtained with this technology due to the thick receiving layers employed.
There remains a need to reduce the possibility of sticking of image receiver elements with donor elements when images are transferred at high temperatures without loss in desired imaging properties. In addition, it would be desired to provide such elements using aqueous-coated formulations so that solvent coating can be minimized. Thus, it would be advantageous to provide an aqueous-coated dye receiving layer that enables high-speed printing without sticking problems. It would also be advantageous if the aqueous dye receiving layer technology could also provide high printing density and be used to provide water-fast prints.